1. Field of the Invention
The present invention relates to a fluorescent tube which is preferably used as a light source of a display device with a backlight.
2. Description of the Related Art
A liquid crystal display device including a liquid crystal display panel, which is cited as an example of a display device, generally has an illuminating device for a display device arranged behind or at a display surface side of the liquid crystal display panel. The illuminating device for a display device includes a fluorescent tube such as a cold cathode tube that is a light source, and controls the properties of light emitted from the fluorescent tube and projects the light toward a rear side of the display panel. The projected light passes through or is reflected by the display panel, making an image displayed visible on a front side of the display panel.
FIG. 11 is an exploded perspective view illustrating relevant parts of a structure of a generally-used conventional liquid crystal display device. A liquid crystal display device 20 includes a bezel 2, a display panel 3 and an illuminating device 4. The bezel 2 is a member that defines a frame of the display panel 3, and the display panel 3 is made by bonding two panels of glass to seal in a liquid crystal therebetween.
The illuminating device 4 includes a frame 5, optical sheets 6, fluorescent tubes 21, a reflector 10, a backlight chassis 11, side holders 12 and an inverter circuit board 13. The frame 5 is shaped like a picture frame and secures the optical sheets 6 to support surfaces of the backlight chassis 11 and the side holders 12. The optical sheets 6 are for controlling the properties of light which enters the display panel 3 from the fluorescent tubes 21, and are made of, for example, a diffusion sheet, a lens sheet, a polarizing reflection sheet and a diffusion plate.
Both ends of the fluorescent tubes 21 are inserted into electrode part holders 15 and 16 so as to be secured to the backlight chassis 11. The reflector 10 which is laid under the fluorescent tubes 21 is for reflecting the light emitted from the fluorescent tubes 21 toward the display panel 3. The backlight chassis 11 and the side holders 12 are members that define a fluorescent tube housing which houses the fluorescent tubes 21 in parallel. In this case, the backlight chassis 11 is made from a metal plate material, which is subjected to plate metal processing so as to be shaped like a box, and constitutes a bottom portion and side-wall portions at the longer edges of the fluorescent tube housing. The side holders 12 are members made from a white resin material and constitute side-wall portions at the shorter edges of the fluorescent tube housing.
On the rear surface of the backlight chassis 11, the inverter circuit board 13 which generates high pulse voltage to drive the fluorescent tubes 21 and an inverter circuit board cover 13a which is arranged to cover the inverter circuit board 13 are placed. In addition, a control circuit board 14 which controls the display panel 3 and a control circuit board cover 14a which is arranged to cover the control circuit board 14 are placed on the rear surface of the backlight chassis 11.
Incidentally, as a prior art literature relating to the present invention, Japanese Utility Model Application Unexamined Publication No. Hei 05-4133 and Japanese Patent Application Unexamined Publication No. Hei 07-20463 are cited.
As shown in FIG. 12, in the illuminating device 4 for display device, the fluorescent tube 21 that is the light source employs a one-side high-voltage driving method with the use of the inverter circuit board 13 in which an electrode part holder 15 side is set to be a high-voltage driving side 8, and an electrode part holder 16 side is set to be a low-voltage driving side 9. The backlight chassis 11 made from metal usually is at a ground potential. When the thus-configured fluorescent tubes 21 are driven, the backlight chassis 11 functions as an adjacent conductor and stray capacitances “Cs” are generated between the fluorescent tubes 21 and the backlight chassis 11. Accordingly, a current passing through the fluorescent tubes 21 partly leaks to the backlight chassis 11 functioning as the adjacent conductor so as to be a leakage current “is”. Generally, the leakage current “is” increases as the lengths of the fluorescent tubes 21 increase, resulting in current differences between the high-voltage driving side 8 and the low-voltage driving side 9, and thereby a luminance gradient occurs in the fluorescent tubes 21 such that the luminance becomes lower from the high-voltage driving side 8 toward the low-voltage driving side 9.
In recent years, accompanied by increases in the size of a screen for a liquid crystal display device used in a liquid crystal television set, the lengths of fluorescent tubes that are light sources increase to cause such a problem that the luminance gradient in the fluorescent tubes as described above appears as a brightness difference between right and left portions on the display screen.